In ignition coils, it is necessary to guide a high tension winding wire from an upper end former of a compartment which carries one winding section towards an end former defining the next compartment, and to do this in such a way as to guarantee dielectric consistency between the two neighboring sections.
With this in view, it is known to arrange an insulating compartment between the two successive winding compartments, with the insulating compartment guaranteeing high insulation between the winding compartments and at the same time serving as a conduit for the winding wire joining the two winding compartments. For that purpose, the walls bounding the winding compartments are provided with slots. These slots allow the winding wire to pass from one high tension winding section to the next, in such a way that after one winding section has been fully wound, the wire can be taken through a slot in the wall of the winding compartment concerned and into the insulating compartment, and thence, via the base of the latter, into the next winding compartment so that the forming of the next winding section can be commenced.
In order to improve the insulating effect still further, it is arranged that the slots in the walls should be offset from each other by 180 degrees about the axis of the insulating body on which the winding is wound. An ignition coil which employs such high tension windings is described and shown in French published patent application No. FR 2 326 769A.
When the secondary sub-assembly is encapsulated in an insulating material, for example polybutyleneterephthalate (PBT), the incompatibility of this material as regards its adhesion to the material of which the insulating support member is made, and to the enamel on the high tension winding wire, means that the plastics encapsulating material does not perform a mechanical function, namely that of securing the turns of each winding section with respect to each other and securing these sections in their respective compartments. Consequently, the ignition coil may tend to suffer rapid deterioration. However, the use of PBT is still recommended because of its other qualities, such as the ease with which it can be moulded, its fluidity which allows it to penetrate easily into narrow spaces, and so on.
In order to overcome the disadvanteges set out above, one of the solutions currently used in practice involves, before encapsulation, the impregnation of the whole coil assembly by pouring a resin into the interior of the housing. This process is somewhat delicate, since the integrity of the resin, in both mechanical and dielectric terms, depends essentially on the achievement of high precision in its composition, in the temperature at which it is flowed into the housing, and in the time which it is necessary to allow for it to cure. The equipment for performing this operation is consequently expensive, especially since this equipment must be quite large since the curing time of the resin is very long, being of the order of several hours.